This invention relates to electrostatic imaging and more specifically to the simultaneous development of electrostatic latent images and the removal of the residual toner images from a support surface
Generally, the xerographic process is performed upon a xerographic plate comprising a layer of photoconductive insulating material upon a conductive backing. The surface of the plate is uniformly charged and then exposed to a light and shadow image pattern. The photoconductive plate discharges in the exposed areas proportionally to the intensity of the radiation reaching the exposed area, thereby creating an electrostatic latent image on the surface of the photoconductive layer corresponding to the light and shadow image pattern projected upon the plate. The electrostatic latent image is then developed by contact with an electroscopic marking material called "toner." The electrostatic latent image which has been developed by contact with toner is then referred to as the "toner image" or "developed image." This developed image may be fixed on the xerographic plate itself, or it may be transferred to paper or other material, and the transferred image may be fixed on said other material. However, after the developed image is transferred to another base material, there may still be and there typically is, a residual image of toner particles adhering to the surface of the photoconductive layer. If this residual image is not removed before the plate is resused, portions of the residual image may be transferred and fixed to any new copy which is made from the same plate.
To control and prohibit subsequent transfer of the residual images in such automatic apparatus, it has been found to be commercially expedient to develop and clean in sequence and at separate locations around a rotating drum or on a xerographic plate and to neutralize the residual charge on the plate and on the residual toner after the toner image is transferred and before the plate is cleaned and recycled. Neutralizing a xerographic plate after transfer with an oppositely charging corona or lamp allows the residual toner particles to be more readily brushed off the surface by any suitable means such as a rotating brush or stationary blade. Once the surface of the xerographic plate is cleaned, the entire xerographic cycle may be started again on the same plate.
Prior art cleaning methods include a brush with bristles which are soft and of suitable triboelectric characteristics, and yet sufficiently firm to remove residual toner particles from the xerographic plate. Also, used are webs or belts of soft fibrous materials or tacky materials, and pliable cleaning blocks with a beveled edge or blades that contact the photoreceptor surface.
In spite of the successes that have been achieved in cleaning the prior art solutions to the problems in the development and cleaning steps in the xerographic process are not entirely satisfactory. For example, cleaning still typically requires bulky apparatus and a separate and distinct cleaning station. Experience has shown that the greater the number of appartus stations necessary to carry out the xerographic process, the greater the danger of toner powder escaping throughout the mechanism and dusting the operating apparatus. Also, typically development and cleaning must be performed at different areas of the xerographic plate, which requires more apparatus to ensure that that portion of the xerographic plate being used to reproduce the desired image is correctly registered at each of the xerographic stations. Experience in the art of photoconductors has shown that the greater the number of passes necessary to clean or develop the surface of said photoconductor, the fewer the number of cycles through which said photoconductor or xerographic plate can be used with acceptable image quality. The surface of the photoconductor is partially abraded by multiple passes through development or cleaning steps, and scratches in the surface of the plate may mechanically pick up toner particles thereby darkening the background areas of desired images. In addition, increased numbers of passes through development or cleaning stations tend to increase toner consumption and to impair toner concentration in the developer system. Each of these effects contributes to reduced image quality in the prior art system.
It should be noted that a cleaning failure, failure of the cleaning station to remove untransferred toner from a previous copy, can result in positive image ghosting on the next copy. Also, a more catastrophic failure can occur if uncleaned toner migrates under the charge corotron, is charged to a high potential, and begins to pull developer bead out of the developer housing.
U.S. Pat. No. 3,649,262, assigned to the same assignee as the present invention, discloses a system for removing residual toner images from electrostatographic image support surfaces and simultaneously developing an undeveloped electrostatic latent image on essentially the same area of the surface by cascading developer along the image support surface of the plate. A difficulty with U.S. Pat. No. 3,649,262 is that high density images are not cleaned and too low of density images are difficult to reproduce. For example, FIG. 3 in U.S. Pat No. 3,649,262 illustrates an operating window or range of operation as function of the of charging voltage and relative exposure illumination. In particular, because of cascade development, the degree of developed image density is limited.
It would be desirable therefore, to provide a simultaneous cleaning and developing system that overcomes much of the prior art difficulties. It is an object, therefore, of the present invention to provide a new and improved simultaneous development and cleaning system. It is still another object of the present invention to provide a conductive twocomponent developer that simultaneously cleans toner from the photoreceptor surface and develops a relatively high or low density image. Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularly in the claims annexed to and forming a part of this specification.